Yom TovHalachos of Kashering

When food is cooked or soaked in a utensil, the utensil is assumed to absorb some amount of the food. Therefore, if non-kosher food was cooked in a utensil, one may not cook any food in it; if chametz was cooked in it, one may not cook food in it for Pesach; and meat utensils may not be used for dairy. Kashering refers to any method which purges the absorption within a utensil, thereby allowing it to be used for other purposes.

A. Determining the Required Method of Kashering

If a utensil was used for non-kosher food, or with meat and milk, it requires[1] ע' סמ"ק (מצ' קצח) מנה הגעלת כלים למצוה.

ובטעם למה אין מברכים עליו, כ' כנה"ג (סי' קכא, הגהת הטור י"ג) משום דאינו מצות עשה אלא סילוק של איסור לא תעשה שלא לאכול נבילות, והוי כניקור חילב, עכת"ד. אולם, לדעת הסמ"ק הוי מ"ע, ואפי' הכי אין מברכים, וי"א משום שאינו מצוה חיובית כיון שאינו חייב להשתמש בכלים אלו דאי בעי משתמש בכלים חדשים (כ"כ אורחות חיים הל' חמץ ומצה סי' צה,). אבל השדי חמד (?) הקשה על טעם זה, דהא גם מצות ציצית אינו מצות חיובית, דאי בעי אינו לובש טלית בת ד' כנפות, ואפי"ה מברך עליו. וראיתי בספר סידור פסח כהלכתו (גרוסמן עמ' ג) לחלק, דבהגעלה אינו מוסיף כלום על הכלי, אלא מפריד האיסור ממנו, אבל בטבילה, המצוה הוא לעשות פעולה חיובית על הכלי, ולכן יש אפשרות לברך עליו. kashering before it may be used. The custom is not to kasher items which were used with meat in order to use them with milk, or vice-versa. One also must kasher items used with chametz before they may be used for Pesach.

There are three ways to kasher: hagala, libun kal, and libun gomur.

The method by which an item is kashered depends on how it became treif (or milchig or fleishig).

In any of the following situations, only hagala is required[2] לדעת שו"ע הרב (תנא?), כל דבר שנאסר מבשר בחלב, צריך רק הגעלה. :

1)     The taste which one is trying to purge is kosher but is merely milchig or fleishig[3] ש"ך (קכא, ח) (e.g., kashering a milchig pot to make it pareve). However, if the taste is milchig which was absorbed into a fleishig utensil, see next paragraph.

2)    The absorption of meat into the milchig utensil (or vice-versa) happened when the utensil was not ben yomo of its normal use[4] ? דהוי היתירא בלע (e.g., one baked chicken on a milchig pan which had not been used for milk in the last 24 hours). (Obviously, this leniency is only applicable when the absorption was from kosher food, not if it came from non-kosher food.

3)    The transfer of taste happened with a liquid medium[5] ש"ך (קכא, ח) ע"פ פמ"ג (מש"ז תנא, טז) (e.g., chicken fried in oil in a milchig pot)

4)    The item which caused it to became treif is a liquid[6] רעק"א בדעת הש"ך הנ"ל (e.g., milk heated in a fleishig pot)

5)    The absorption happened via a hot food which was already removed from the kli rishon[7] כגון סכינים (תנא, ג) (e.g., a hot steak placed on a milchig plate)

However, if none of the above apply, the item will usually require libun gomur. For example, if one roasted chicken on a milchig, ben-yomo pan, the pot requires libun gomur.

Some maintain that any absorption of meat and milk can always be purged through hagalah and libun is never required[8] שו"ע הגר"ז (תנא, כה), וכ"כ בשבט הלוי (ו, נו) שהוא מסתבר מאד. ; one should consult his Rav.

If the absorption happened via pouring hot liquid from a kli rishon, one may kasher by pouring hot[9] ובדבר שנשתמש בו ע"י עירוי, אפי' מים שהיד סולדת בו מהני (מ"ב תנב, ח) water from a kli rishon[10] ? . Therefore, sinks and counters may be kashered via pouring hot water on them. However, if the non-kosher absorption happened while the item was directly on the fire, it should not be kashered via pouring.

If the absorption happened via steam, they may be kashered via steam[11] ? אג"מ? . This applies to microwaves; see later.

B. Which Items and Materials May Be Kashered

Metal. Metal items may, depending on the manner by which they absorbed bliyus, be kashered via hagala or libun.

Glass and Similar Items. Glass does not need kashering from meat, milk, or non-kosher use in order to use it for kosher, provided it is cleaned and scrubbed well before use^[12] ע' אג"מ (או"ח ה, לב), דלא החמיר רמ"א (תנא, כו) אלא לענין פסח . However, it is recommended to buy separate glass utensils if they are to be used with hot milk or hot meat. For Pesach, glass used for chametz may not be used, and glass may not be kashered for Pesach[13] רמ"א (תנא, כו), וע' שו"ע (שם) דלא בלעי כלל . Corelle, Pyrex, and crystal have the same status as glass[14] Star-k.org בשם הגר"מ היינעמאן .

Plastic. Plastic items may be kashered from meat and milk or non-kosher bliyus in order to use them for kosher. Plastic may not be kashered from chametz for use on Pesach[15] ע' אג"מ (?) שיש להכשירם ו-(?) שאין להכשירם, והגר"ש איידער אמר שדעת הגרמ"פ דמותר להכשירם משאר איסורים אבל לא לפסח . However, some opinions maintain that plastic cannot be koshered. Therefore, one is permitted to kasher items which only questionable absorbed a non-kosher taste, such a microwave, but not items which directly touched hot, non-kosher food. 

China. China, earthenware, and ceramic items may not be kashered in any manner.

From Meat to Milk. The custom is not to perform hagala on fleishig items in order to use them as milchigs, or vice-versa[16] מ"ב (תנא, יט) . However, it is permitted to kasher in order to use it as pareve[17] דרכ"ת (קכא, נט) (such as if one’s pareve utensil accidentally became fleishig), or to kasher it via libun[18] דרכ"ת (קכא, נט) (e.g., kashering an oven from milchig to fleishig use). For more cases it is permitted, see footnote[19] מותר להחליף המין אם מכשירו לפסח (מ"ב תנא, יט) [ולא כתבתי כן בפנים כי נ"ל שהנשים לא יסכימו להגעיל כלי שידוע להם שהוא בשרי לעשותו חלבי], או אם מטריפו קודם ההגעלה (מ"ב תקט, כה, בשם פמ"ג) [אבל אין זה רעיון טוב, כי אפשר לבוא לידי מכשיל אם מבשל בו בשר כשהוא בן יומו מבישול החלב, שעובר בזה איסור בישול בשר בחלב]. וגם בשעת הדחק נראה דיש להקל, כ"כ פמ"ג (אש"א תנב, יג). וע' שו"ת להורות נתן (ח"א ס' לה) מתיר להגעילו כשקנה כלי מאחר שהשתמש בו חלב והקונה רוצה להשתמש בו בשר. וע' ערוך השלחן (תקט, י) ופר"ח (יו"ד צז, א) דמותר להחליף מבשר לחלב או איפכא, וכן נראה דעת הט"ז (יו"ד קכא, ב). .

Any item which requires hagala can also be kashered via libun kal or libun gomur[20] ב"י (תנא ד"ה והליבון) בשם המרדכי .

C. How To Kasher

Hagala (recommended for cutlery and other small items- see earlier for cases where hagalah does not work)

A.    After the item becomes treif, one should scrub it clean of all food residue, so it will be clean for kashering[21] . If one purchased a non-kosher item, rust and other residue must be either scrubbed off or burnt off[22] מ"ב (תנא, כב) . If one had an item which became treif, rust which was there before it became treif need not be removed[23] שו"ת אבן ישראל (ז, כא) .

B.    The item may not be used with anything hot (or sharp) for 24 hours before it is kashered[24] ?

ובשעת הדחק, העולם סומכים על החזון איש (?) שיכול ליתן דבר הפוגם לתוך המים כדי להכשירו תוך מעל"ע, משום דכשיבוא הבליעה לחוץ, נעשה נ"ט לפגם, ואח"כ אפי' אם יכנס עוד לתוך הכלי אינו אוסר. .

C.    One should heat a large pot of water to a rolling boil[25] מ"ב (תנא, לו). אמנם, אם נבלע הטעם ע"י מים חמים שלא על גבי האש, אין צריך להגעילו ע"י האש ובדיעבד מועיל כל זמן שהיד סולדת בו, אבל המנהג להגעילו על האש (פמ"ג מש"ז ט). וגם, אם נבלע רק בליעה של היתר, אין צריך להגעילו על האש (שו"ע הגר"ז ס"ק כה, וכ"כ בשבט הלוי (ו, נו) שהוא מסתבר מאד. .

D.    While the water is boiling, the item should be dropped in, and then removed with tongs after the water re-boils again[26] מ"ב תנב, ? (as it will momentarily stop boiling when the temperature drops due to the utensil), approximately four seconds later. One should not turn off the fire while the item is in the pot. When kashering multiple items, one should remove each item after three seconds, and ensure the water is boiling before putting in each new item[27] מ"ב .

E.     As each item is taken out of the hagala pot, it should immediately be placed into cold water[28] תנב,ז. ואם לא עשה כן, בדיעבד אין לחוש .

Hagala By Pouring (permitted only for sinks or items which became non-kosher by pouring non-kosher food onto them- see earlier for cases where this hagalah does not work)

A.    The sink must be cleaned of all residue. The strainer should be discarded, as it is difficult to clean it fully.

B.    It may not be used for anything hot for 24 hours before kashering.

C.    It must be completely dry before kashering.

D.    One should heat a large pot of water to a rolling boil.

E.    Boiling water should be poured on every area of the sink, with a direct, uninterrupted stream from the pot reaching everywhere. It should be poured slowly, starting from the edges in a circular motion, to ensure that water reaches everywhere.

F.    After kashering, one should spray cold water from the faucet on the entire sink.

Libun kal in an oven (recommended for pots or pans which don’t fit in your largest pot- since this only works in cases where hagala would work, see earlier for cases where hagalah does not work)

A.    The item must be cleaned of all residue[29] אמנם, אם עושה ליבון קל כנגד האש, אין צריך לנקותו כיון שהכל נשרף .

B.    The item should not be used for anything hot or sharp for 24 hours before koshering[30] ע' פמ"ג (מ"ז תנב, ד) נראה דנסתפק אם צריך להיות אינו בן יומו .

C.    The item must be heated so the entire item reaches 450[31] ? degrees Fahrenheit. One may preheat oven to 550 and place the item inside for twenty five minutes[32] כ"כ חיים שטייער בספרו מאור החיים על הגעלת כלים בשם CRC . והר"מ יארדלי אמר בשם הר' שלמה מילר, 550 לדקה אחת. , or on 500 degrees for an hour[33] כתב לי star-k.org, שיכול לעשות 500-550 מעלות למשך 45-60 מינוט. .

Libun gomur (works for all items; required for certain cases, see earlier)

A.    Libun gomur requires heating the item to 800[34] ? degrees Fahrenheit, which can be performed by placing it in the oven while performing a self-clean cycle.

On items with a temperature control, one may kasher it by heating it on its highest temperature for long enough for the entire item to reach that temperature (approximately an hour)[35] בשם הגר"א קוטלר ; one should consult a Rav whether this is appropriate for each specific item.

In order to kasher one’s largest pot via hagala, one must make a rim around the top of it and allow it to overflow with boiling water[36] תנב,ו . When one kashers a large pot for the purpose of kashering other utensils inside it or via pouring from it, one does not need to overflow it[37] מ"ב תנב,כט .

C. Kashering for Pesach

Sinks. To kasher, one should fill a large Pesachdik[38] כן נוהגים. וע' מ"ב (שעה"צ תמד, ד) כ' ואין לערות רותח לתוך כלי חמץ מכלי חמץ, עכ"ד. ונראה טעמו או משום זיעה או משום שקילוח אוסר הכלי שהוא למעלה. והטעם שנוהגים להקל, דהא הכלי שמערים לתוכה לא הוי בן יומו, וגם שלא בלע אלא ע"י עירוי כלי ראשון ולא כלי ראשון ממש, וגם שיש מחלקת אם בבשר בחלב העירוי יכול לאסור הכלי דלמעלה, עמ"ש בהל' בב"ח {?}. ע' שעה"צ (תנב, טו) דכן נוהגין להכשירו קודם. pot with water, and follow the above-mentioned procedure.

Stove tops – To kasher a gas stovetop, the grates[39] אע"פ שבכל השנה אין צריך להגעילם בין בשר לחלב, בפסח מחמירים בחצובה (תנא, ?) should be placed in the oven and heated on the highest setting for 90 minutes[40] דדי בליבון קל כיון שאין מכשירים אלא משום חומרא דפסח (?) , or while it self-cleans.

For electric burners, turn on the burners until they glow.

From StarK.org:

Kashering a Glass, Corning, Halogen, or Ceran electric smoothtop range for Pesach use is a bit complex. To kasher the burner area, turn on the elements until they glow. The burner area is now considered kosher for Pesach. However, the remaining area that does not get hot is not kashered. The manufacturers do not suggest covering this area as one would a porcelain top, as it may cause the glass to break. Real kosherization can be accomplished by holding a blow torch over the glass until it is hot enough to singe a piece of newspaper on contact with the glass. However, this too may cause the glass to shatter and is not recommended. As the area between the burners cannot practically be kashered, it would be wise to have a trivet on the open glass area to move pots onto. In addition, it would be wise to place a metal disc, approximately 1/8 of an inch thick, on the burner area to raise the Passover pots above the rest of the glass surface. (Caution: This disc should not extend beyond the designated cooking area.) This will help in case the pot boils over, sending a trickle of hot liquid that would serve as a connector from the Passover pot to the non-Passover stovetop. (Note: Cooking efficiency may be compromised when using metal disc.)

After koshering the heating element, all other surfaces on the stovetop should be covered with heavy-duty aluminum foil. The hood should be covered with aluminum foil the prevent the steam from coming in contact with it[41] .

If an oven has a self-clean option, one may self-clean it and no prior cleaning is required. If the oven does not have this option, the oven must be scrubbed clean of all residue, preferably using a caustic oven cleaner (e.g., Easy Off), and then set on its highest heat for 90 minutes[42] {ע' אג"מ מחלק בין פסח ובב"ח} .

Countertops – Countertops made of metal or unsealed granite, may be kashered by pouring hot water on every part of it, slowly and methodically. Other countertops, such as those made of sealed granite, Quartz, Formica, or tile, must be covered with a thick plastic or wood.

Shelves – Pantry and refrigerator shelves should be covered with a shelf liner.

D. Kashering the rest of the year (not for Pesach):

One may use the same drinking glasses during meat and milk meals, provided they are washed clean of residue beforehand, and do not require kashering[43] ע' אג"מ (או"ח ה, לב), דלגבי כל השנה, אין זכוכית בולע, דלא החמיר רמ"א (תנא, כו) אלא לענין פסח. .

One who wants to heat up food in a non-kosher oven or microwave may double-wrap the food and then heat it. If it was heated in a microwave without a covering, a Rav should be consulted. If, accidentally, it was heated in an oven without a covering, the food may be eaten.

One may kasher a microwave in the following manner: first, ensure the microwave walls and plate are clean of all residue. Then, insert a full cup of water and heat it for several minutes until all the steam has evaporated[44] דמגעלין פלאסטיק לצורך כל השנה חוץ מפסח, כן העידו כמה פוסקים (?) בשם הגר"מ פיינשטיין. . After doing so, one may even put food directly on the glass plate.

Dishwashers should preferably not be kashered and one should buy a new one[45] לאחר י"ב חדשים, יש להקל במקום הפסד מרובה .

To kasher an oven, one should self-clean it. For ovens which do not have this option, one should remove all large pieces of residue from the oven (no scrubbing required) and set it on its highest heat for 90 minutes. This kashers all surfaces and the racks.

Stovetop grates do not require kashering, even if they were used for non-kosher food[46] אג"מ (יו"ד א, נט) . However, one must ensure they are clean of non-kosher residue, or of milk residue before using them for meat, before use[47] אג"מ (או"ח א, קכד) .

An outdoor grill (charcoal or gas) requires libun gomur on the parts which directly contact the food, which are the grates, and the part of the grill cavity which is level with the grate. This can be accomplished by sandwiching coals between the grates and then setting them on fire. The empty grill cavity can be kashered by filling it with coals, setting them on fire, and allowing them to burn for 40 minutes. In the case of a gas grill, the empty cavity can be kashered by cleaning the cavity from all residue, closing the hood, and setting it on the highest heat for 40 minutes[48] Star-k.org (הר' צבי רוזן) .

If one purchases a house from a non-Jew, so the countertop will be used on a permanent basis, he should first clean it of all residue, and then kasher it by pouring boiling water on the entire surface. This method works, for non-Pesach use, for granite (sealed or not), Formica[49] שהוא מין פלאסטיק. וגם ה-sealant של שיש הוי פלאסטיק ויש להכשירו לכל השנה אבל לא לפסח. , Corian and quartz[50] Quartz נעשה מאבנים שחוקים שנדבקו יחד ע"י resin, והגר"י בעלסקי (הו"ד ב-oukosher.org) אמר שיש להקל להכשירם, שה-resin הוי דבר מועט ודינם ככלי אבנים, ומותר להכשירם לכל השנה (אבל לפסח אמר שלא להכשירם). .

A table purchased from a non-Jew does not require kashering, but one must thoroughly wipe it before use to ensure it is clean from all residue.